Many high-speed, automatic strapping machines have been developed, such as those disclosed in U.S. Pat. Nos. 3,735,555; 3,884,139; 4,120,239;, 4,312,266; 4,196,663; 4,201,127; 3,447,448; 4,387,631; and 4,473,005. As disclosed by the devices in these patents, a conveyor belt typically conveys a bundle at high speed to a strapping station where straps are automatically applied before the conveyor belt moves the strap bundle away from the device. Current machines are able to strap approximately 40 to 50 bundles per minute. However, it is desirable to further increase the speed of such strapping devices to thereby provide enhanced throughput.
Typical strapping machines employ an initial or primary tensioning apparatus that provides an initial tensioning of the strap about the bundle. A secondary tensioning apparatus thereafter provides increased or enhanced tension of the strap. Thereafter, a sealing unit or head seals the strap, typically through the use of a heated knife mechanism, to complete the bundling operation.
Prior strapping devices relied exclusively on mechanical assemblies, such as multiple cam and follower mechanisms, piston driven linkages, etc. for timing. Such mechanical mechanisms can provide quite rapid strapping of certain bundles. However, if bundles of various sizes, and consisting of various types of material, are to be bundled, such mechanical strapping devices can excel in strapping, only one size bundle objects, while poorly strapping another size bundle or a bundle of different objects. Such mechanical, or electromechanical, machines are unable to automatically adjust for differing size bundles or bundles of different objects that are rapidly sent to the machine. Additionally, such mechanical devices may be unable to effectively bundle objects at speeds in excess of 60 bundles per minute. Importantly, both the primary and secondary tensioning devices are unable to reliably operate at such high speeds.
In general, the strapping machines currently on the market use traditional electromechanical components such as clutches, brakes, V-belts, etc. for power transmission. The widespread use of servo controls in other industries, however, now makes their use in strapping machines an economically and technically viable alternative to these traditional electromechanical devices.
Traditional servo drive architecture, however, typically involves the use of a PLC (programmable logic controller) platform and so called "smart" servo drive cards to drive the servo motors. Unfortunately, this architecture imposes significant delays in the control program which are not acceptable at high speeds. The PLC based system essentially operates in a master/slave relationship with a main central processing unit ("CPU") issuing a command to the drive card and the drive card executing the command, no real time link between the CPU and the card is provided. Without a real time link, the control system is inflexible and the CPU does not have complete control over the move routines sent to the servo motors.